The present invention relates to energy absorbers with crush boxes and back straps for stabilizing the crush boxes for improved energy-absorbing functionality.
Polymeric energy absorbers are often used on a face of metal bumper beams to provide energy-absorption during a vehicle crash (prior to deformation of the metal beam). Recently, many energy absorbers have incorporated geometrically-shaped tubular crush boxes configured to crush in localized areas with optimal and predictable energy absorption. Crush boxes typically have four (or more) relatively planar walls that extend parallel a direction of expected impact to form a tubular shape, and have an end wall connecting the planar walls to form a box-shaped structure. The walls all support each other to promote a predictable crush-type collapse (with multiple bends in each wall during collapse) for maximum energy absorption. A problem is that, as crush boxes are elongated in a sideways direction perpendicular to the direction of expected impact, their side walls are also elongated and soon become unstable. Specifically, as the side walls are elongated, a stability provided by adjacent side walls and the end wall is decreased, especially in the center of the elongated side wall. Concurrently, energy absorption by the elongated side walls during an impact drops off, because the side walls, especially at locations away from the adjacent side walls, begin to kick outward instead of crushing. Restated, during impact, the crush box's elongated side walls (which form a C-shaped cross section with the associated end wall) tend to spread apart (i.e., the top side wall bends upward, and the bottom side wall bends downward), resulting in a substantial reduction in impact energy absorption.
However, there are reasons to elongate crush boxes in a sideways direction (i.e., in a direction parallel a length of the bumper beam on which the energy absorber is positioned). For example, it is desirable to provide continuous support for fascia across a face of the energy absorber, without interruption of the face surface. This is not possible where adjacent crush boxes are spaced apart since there is a gap between adjacent (spaced-apart) crush boxes. Further, it is desirable to provide a more continuous support structure cross a face of the energy absorber so as to provide a more uniform surface if a pedestrian is struck. Still further, there is a desire to reduce the complexity of energy absorbers, such as by reducing a number of the individual crush boxes (and reducing the number of side walls and reinforcement ribs that must concurrently be made). Reducing the number of walls in an energy absorber simplifies tooling and also increases moldability due to the reduction in complexly-shaped surfaces in the mold. Also, protrusions in a die that are bound on four sides (e.g., the die component forming the inside cavity of a crush box) are difficult to cool since it is difficult to route cooling lines into and out of the protrusions.
Thus, an energy absorber and related method is desired solving the aforementioned problems and having the aforementioned advantages. Specifically, an energy absorber is desired having elongated crush boxes, but with reduced tendency of the crush boxes to “spread” unacceptably during an impact, resulting in unacceptably low energy absorption.